1. Technical Field
This invention is a process for manufacturing composite articles, and more particularly a process for improving closed vacuum assisted molding techniques used in the fabrication of composite articles.
2. Description of the Prior Art
Various manufacturing methods are used in the fabrication of composite articles. These various manufacturing methods are broadly classified as closed mold and open mold processes. Closed mold processes can be cost effective when molding relatively small articles, but becomes cost prohibitive when molding large articles, such as boats, because large and intricately matched tooling becomes too expensive. Therefore, most large composite articles are currently manufactured using open molds.
Open mold processing used in the manufacture of composite articles involves the positioning of reinforced fiber material in a single open mold cavity and spraying or flow coating the reinforcements with liquid curable resin. A variation of this method involves chopping fiberglass in front of the resin spray stream. In this case the reinforcements and curable resin are deposited simultaneously in the mold. A significant drawback to open molding techniques is that during the resin deposition stage large amounts of hazardous air pollutants (HAPs) are emitted, which is a matter of increasing concern to the Environmental Protection Agency (EPA). Therefore, the use of open molds in the fabrication of composite articles is in danger of extinction unless the emission of HAPs can be significantly reduced. Already, Maximum Achievable Control Technology (MACT) Standards are being imposed on the industry to curb HAP emissions. A solution for reducing HAPs, which is well known in the art, is to enclose the open mold apparatus within a vacuum bag during resin infusion. This process is sometimes referred to as vacuum assisted resin transfer molding (VARTM).
The VARTM process typically requires that the reinforced fiber material be positioned over the surface of an open mold. A vacuum bag comprised of flexible polymeric film material is then placed over the reinforcing fiber material and sealed. Resin is introduced into the interior of the vacuum bag after vacuum pressure is applied to draw the resin through the reinforcing fiber material. Once the resin is fully cured, the vacuum bag must be removed from the molded article and either discarded or reused. A significant advantage to the use of a vacuum bag is that HAPs generated from resin transfer are reduced.
One important factor, which must be carefully monitored when using a vacuum bag in resin transfer molding is the formation of air pocket, voids in the composite. Air pockets can form between the vacuum bag and/or within the reinforced fiber material, causing both structural deficiencies and aesthetic damage to the composite article. Additionally, wrinkles can form on the surface of the vacuum bag and transfer to the surface of the reinforcing fiber material, causing part defects. Although slowing down the evacuation process can reduce the occurrence of air pockets and wrinkles, it also results in reduced production rates, and therefore increased costs. Any air pockets, voids or wrinkles which do form must be manually eliminated via a time-consuming manual smoothing process, which can cause perforation of the vacuum bag or seam separation if not executed properly, and also slows production rates. Any leaks in the bag will cause air to be introduced into the resin stream. This problem causes a quality issue commonly called xe2x80x9cbubble trailsxe2x80x9d. Such defects that are not corrected during the molding process require costly reworking.
Various alternative approaches have been proposed to improve on the composition of the vacuum bag in order to reduce the formation of air pockets and wrinkles and the attendant defects. For example, U.S. Pat. No. 5,129,813 to Shepard discloses a pre-formed vacuum bag comprised of a non-porous material having a three-dimensional pattern, which allows the resin to flow through a series of interconnected channels, therefore minimizing the entrapment of air. Significantly it has been found that vacuum bags manufactured in accordance with the aforementioned prior art can be labor-intensive, and therefore costly. First, care must be taken to ensure that the vacuum bag is capable of a controlled collapse during evacuation; otherwise air pockets and wrinkles can form. Moreover, the pre-formed vacuum bag must be carefully constructed such that no tension exists in the non-porous material. Finally, reuse of the bag is discouraged, therefore resulting in unwanted solid waste.
It is also known in the prior art to use a sheet of textured film and tape to form an envelope as an alternative to a costly pre-formed vacuum bag. For example, U.S. Pat. No. 5,837,185 to Livesay et al. discloses this technique. One important factor, which must be carefully monitored when employing this method, is that the mold, seal and envelope must be thoroughly checked to ensure that no air leaks are present. Leaks could result in the formation of air pockets, and cause defects in the laminates or HAPS emissions into the atmosphere. Additionally, as with the aforementioned preformed vacuum bag, the textured film and tape are not reusable, therefore generating unwanted solid waste.
A reusable vacuum bag as an alternative to the wasteful non-reusable methods discussed herein is disclosed in U.S. Pat. No. 5,716,488 to Bryant. Significantly, it has been found that reusable vacuum bags can be costly and time-consuming to fabricate. First, costly silicone rubbers capable of withstanding repeated applications must be used. Second, the fabrication of the reusable vacuum bag requires a labor-intensive process involving elaborate piecing and seaming to protect against air leaks. Third, a different reusable vacuum bag must be made to allow for different sizes and shapes of composite articles. Fourth, reusable vacuum bags require regular maintenance such as repairing, cleaning and storing. Finally, the operational life of the reusable vacuum bag is limited, and the vacuum bag must be eventually discarded and replaced.
A recent advancement in vacuum bag molding is to incorporate structural framing members prior to the commencement of resin. This adds to the complexity of the vacuum bag configuration and amplifies the difficulty of placing the bag and ensuring that the bag is completely sealed.
Accordingly, it would be desirable to provide a process for the manufacture of composite articles that can protect against the formation of air pockets; thereby significantly reducing the number of damaged parts. It would also be desirable to provide a process, which can prevent the emission of HAPs, without the high material and labor costs, and the generation of solid waste attendant with the prior art. Finally, it would be desirable to provide a process that would allow the unfinished side of the composite article to have a desired finish, eliminating the need for an additional finish to be applied. Commonly, unsaturated polyester gel coat materials are applied to the unfinished side of the composite article. These finishing materials further add to the HAPS emissions during the manufacturing process.
The inventive method concerns a process of transferring resin into reinforcing fiber material used in the manufacture of composite articles. A first step in the method involves positioning at least one layer of the reinforcing fiber material on the surface of an open mold. The reinforcing fiber material typically consists of glass, carbon, aramid, linear polyethylene, polypropylene, and polyester fibers. Subsequently, a sealant layer is applied in liquid form over the reinforcing fiber material to create an airtight chamber encapsulating the reinforcing fiber material between the sealant layer and the mold. After the sealant layer is allowed to cure, a vacuum pressure is applied to the airtight chamber to draw resin through the reinforcing fiber material. For example, the resin can be an epoxy, polyester or vinyl ester resin. According to one aspect of the invention, prior to the first step of positioning the reinforcing fiber material on the surface of the mold, an optional gel coat layer can be applied to the mold surface.
The sealant layer can be applied using a spraying process. For example, a sealant layer spray coated to a thickness of between approximately 5 to 20 mils has been found to provide an acceptable result. The sealant layer is typically comprised of a polyurea or polyurethane elastomeric material.
According to yet another aspect of the invention, the polurethane or polyurea formulation can be modified to in order to add enhanced properties, such as improved chemical resistance, moisture resistance, abrasion resistance, ultraviolet light resistance to provide the desired final finish, thereby eliminating the additional step of painting or applying a gel coat.
According to one embodiment, optimal placement of the vacuum and resin ports can be achieved by utilizing a special purpose nozzle assembly designed to pierce the sealant layer for access into the airtight chamber. The inventive method allows for the special purpose nozzle assembly to comprise the vacuum port, the resin port, or both.
Another embodiment for placement of vacuum and resin ports is to place the ports onto the laminate prior to the application of the bag sealant. After placement, the ports can be sealed to the laminate during the application of the bag sealant. After molding, the exposed protruding tip of the port can be optionally cut flush with the laminate surface.
Yet another embodiment is the method of sealing the flange of the mold. The advantage of the sprayable bag here is that overlaps pleats or seams in the bag do not complicate flange sealing. Resin injection and/or vacuum channels can easily be accommodated by using channel details molded into the flange or by placing a manifold system or resin transfer media on the flange prior to the application of the sprayable bag material.